In his first-ever interview, the sole Australian survivor of the Waco siege has revealed to 60 Minutes what really took place during the 51-day stand-off, and why he believes crazed cult leader David Koresh will one day return to Earth as the son of God.

Cool winds were expected to bring relief tonight for Melbourne and Adelaide, but Sydney and Canberra face another hot one on Wednesday.

Sydney's overnight temperatures are expected to remain at a sticky 26C, not far off the record of 26.3C set in December 1972.

By 9am on Wednesday, temperatures are expected to be well into the 30s and peak shortly after at 38C.

The bureau's senior forecaster Neale Fraser said a southerly change should cool things down late in the afternoon.

"That will bring a dramatic drop in the temperature," he said.

"There also may be some late showers as well. But before then a hot night and another hot day."

To help cope with the hot weather, the Red Cross is urging people to keep an eye on their neighbours.

"Older people, pregnant women, children, those with a disability and people taking medications are among those who are more at risk," a spokeswoman said.

St Vincent's Hospital and workers from the City of Sydney council have been busy providing homeless people with water, sunscreen and details about where to seek shade.

Meanwhile, more than 45 fires were burning across NSW on Tuesday, including two large blazes which threatened dozens of homes near the towns of Abermain and neighbouring Neath, near Cessnock in the Hunter region.

At least 70 firefighters were on the ground while water bombing helicopters helped from above.

With hot and windy conditions forecast to continue, total fire bans have been declared for the greater Sydney region, Illawarra/Shoalhaven, Upper Central West Plains and Lower Central West Plains on Wednesday.

However, fire bans that were in place across northern Victoria and most of South Australia are expected to be lifted.

The SA Country Fire Service said that the threat posed by a fire on 3300 hectares of land near the Nullarbor Roadhouse on the Eyre Highway had reduced.